The AS15531 databus, also known as MIL-STD-1553 or simply 1553, is an approximately 30 year old technology that defines the electrical and signaling characteristics for 1 Mbps communications over an asynchronous serial, command/response digital data bus on which messages are time division multiplexed among users. The transmission medium is a twisted wire cable pair. 1553 specifies all of the electrical characteristics of the receivers, transmitters, and cable used to implement the bus, as well as the complete message transmission protocol. 1553 is designed for high integrity message exchanges between unattended equipment. The messages are generally highly repetitive, and their content and periodicity are all pre-planned.
The United States Department of Defense (“DoD”) requires the use of 1553 as the standard for all inter and intra-subsystem communications on all military airplanes, helicopters, ships and land vehicles. Originally used only in mission avionics, 1553 is now used in flight critical avionics, flight control, weapons, electrical power control, and propulsion control. 1553 was originally published in 1973 for use on the F-16 military aircraft program. The current version of 1553 is MIL-STD-1553B (“1553b”), Notice 2, implemented in 1986.
1553 is generally utilized for hard real time communications, where a message is expected to be communicated over the bus in a deterministic way with known latency and very low probability that the message is not decoded successfully. For these critical communications, the standard specifies a primary data bus as well as a redundant (default) bus, providing communications path redundancy (“dual bus redundancy”). For dual redundant bus applications, 1553 requires that a terminal be capable of listening to and decoding commands on both buses at the same time. A 1553 terminal transmits 1553 signals on only one bus at a time. Redundancy can be extended to more than 2 buses.
MIL-STD-1553B utilizes a primitive Manchester II bi-phase signaling scheme over shielded twisted pair cabling. This modulation scheme is bandwidth inefficient, with most of its signal energy concentrated around 1 MHz. MIL-STD-1553b has little remaining capacity for existing applications and leaves little opportunity to enable additional communication capabilities.
The retrofitting of an aircraft to add new equipment, Line Replaceable Units (LRU's) and/or munitions, including new wiring, is a complex process, which can require many months of modification time and involve substantial expenses. When new digital devices are added to an after market military or commercial aircraft, the addition thereof typically requires new bus wiring or an expanded load on the already heavily loaded aircraft wiring cockpit applications. New devices, that may only require minutes to install, often require an entire airframe to be nearly disassembled to allow new wiring runs to the new devices. Furthermore, the new wiring adds weight to the aircraft and takes up space, which is always disadvantageous in any airframe design, especially for high performance airframes in which maneuverability is important.
Furthermore, new equipment, such as LRUs or munitions, which are retrofitted to an airframe often require high bandwidth data links between the new equipment to points in the airframe where control or monitoring is performed. High bandwidth communications between state of the art digital equipments are necessary.
The Society of Automotive Engineers (SAE) Avionics Systems Subcommittee (AS-1A), in cooperation with SBS Technologies Inc., have investigated the use of Discrete Multi-Tone (DMT) signaling as a possible technology to increase the data transfer capacity of existing AS15531 networks. Their findings are summarized in a white paper entitled “The Use of Discrete Multi-Tone (DMT) Signaling for Data Transmissions on Existing AS15531 Networks”, published on 15 Aug. 1998, which is incorporated herein by reference. Experimental studies have considered only a point-to-point connection of commercial Digital Subscriber Line (DSL) modems over a 100 ft piece of MIL-C-17 AS155531 cabling. This work indicated that the cable becomes surface impedance unstable and lossy at frequencies above approximately 10 MHz (see p. 5, paragraph 4.3). In addition, test results of standard AS15531 couplers used in this work indicated that the couplers had a band-pass capacity of between 2 and 3 MHz. (see p. 5, paragraph 4.4). Simultaneous DSL and AS15531 traffic was observed in the case of a Multi-rate Symmetrical DSL (MSDSL) modem, however MSDSL telecom modems would begin reporting significant number of errors if the AS15531 transactions were scheduled at frequencies above approximately 10 MHz. In the case of an Asymmetrical DSL (DSL) telecom modem tested, AS15531 bus traffic was detected by the modems and was significant enough to reset the modem link. The study fails to indicate the feasibility of an operable system that would allow existent 1553 networks to operate in their multi-drop, dual-redundant architecture, at a data transfer rate above 1 Mbps and signaling frequencies above approximately 10 MHz.
There is a need for an improved 1553 communication system with a data transfer capacity above 1 Mbps that can be overlaid over an existent 1553 network without rewiring.